Method for producing high density sintered silicon nitride (SI3 N.sub.4

ABSTRACT

The specification describes a method for producing high density sintered silicon nitride(Si 3  N 4 ) having a relative density of at least 98%. In a first step, silicon nitride powder is compacted into a desired shape. It is then presintered in a second step, generally, under normal pressure to obtain a presintered body having a relative density of at least 92%. In a third step, the presintered body is subjected to a hot isostatic pressing(HIP) in an inert gas atmosphere of 1500-2100° C. and of nitrogen gas partial pressure of at least 500 atm. Since the presintering does not require any capsule, it is possible to produce high density sintered Si 3  N 4  of complex configurations. As a sintering aid, Y 2  O 3  -Al 2  O 3  -MgO system sintering aid is particularly effective. To improve the strength of sintered Si 3  N 4 , it is effective to add a heat treatment step after the HIP and maintain the temperature of the sintered Si 3  N 4  above 500° C. for a while. Between the second and third steps, the temperature of the presintered body is preferably maintained above 500° C. These temperature controls are effective not only to improve the strength of sintered Si 3  N 4  but also to save the thermal energy and to shorten the production cycle.

This application is a continuation of application Ser. No. 06/312,727,filed on Oct. 19, 1981, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for producing high density sinteredsilicon nitride(Si₃ N₄) having a density equivalent to or higher thanthat of a conventional hot pressed body, which method facilitates theproduction of sintered bodies of complex configurations. Moreparticularly, it relates to a method of further densifying andstrengthening a presintered silicon nitride body, which has in advancebeen formed and presintered into a desired shape by the pressure-lesssintering method or the like, in accordance with the hot isostaticpressing method(hereinafter referred to as "HIP method").

2. Description of the Prior Art

With a view toward improving the thermal efficiency, saving fuel,reducing pollution and lightening the weight of varied power generatingequipments, active research and development work has been carried out inrecent years in the field of equipment which are operated at hightemperatures led by high temperature gas turbines and followed by dieselengines and MHD generators. Development of such equipment is absolutelydependent on the development of high-temperature structural materials.Thus, the development of such materials has been anxiously waited for.Under such high temperatures, conventional heat resistant metallicmaterials do not always exhibit sufficient mechanical strength. Also,from the view-point of saving exiguous natural resourses required forthe production of such heat resistant metallic materials, intensiveresearch is now under way to develop high-temperature structuralceramics using as raw materials Si, Al, C, N and the like elements whichare relatively abundant on the earth.

The development of such high-temperature structural materials has beenrecognized to be also important for the purpose of their applications insuper hard tools and as corrosion resistant materials, and is attractinggreat interests thereon.

Among such high-temperature structural ceramics, silicon nitride(Si₃ N₄)is attracting researchers' interests as one of most promising materialsfor providing sufficient strength, chemical stability and strongresistance to thermal shocks at high temperatures.

Si₃ N₄ has excellent physical properties as described above. Theseproperties stem from the fact that silicon atoms (Si) and nitrogenatoms(N) are united by firm covalent bonds in silicon nitride (Si₃ N₄).However, this nature of Si₃ N₄ means, on the other hand, that it is hardto be sintered and extremely difficult to be formed into a product ofcomplex configurations. As a matter of fact, most of the recent researchin this field has been directed to how to produce formed Si₃ N₄ bodieshaving high strength. However, in view of the present state of the art,it does not appear that a fully satisfactory technique has beendeveloped for the production of such Si₃ N₄ bodies.

More specifically, where high density and high strength are pursued, theshape of a formed body has to be unavoidably limited to simple ones. Onthe other hand, to obtain formed bodies of complex configurations, itsstrength has to be sacrificed to a considerable extent.

Among well-known conventional production methods of formed Si₃ N₄bodies, there are the following four methods: (i) the chemical vapordeposition method(hereinafter referred to as the CVD method); (ii) amethod in which Si₃ N₄ powder is mixed with a sintering aid and thensintered in an N₂ atmosphere of atmospheric pressure or 10 atmospheresor so; (iii) the hot pressing method; and (iv) reactio bonding method.Of the above methods, the hot pressing method(ill) can provide formedbodies of a relatively high density and strength but is stillaccompanied by problems that formed bodies of a complex shape aredifficult to obtain and it is costly to practise.

On the other hand, the reactin bonding method has a merit that complexconfigurations can easily be formed by a suitable conventional methodowing to the use of Si powder as a raw material. However, it isaccompanied by drawbacks that resultant sintered bodies do not have ahigh density and high strength sintered Si₃ N₄ cannot be obtained. Thedensities of sintered Si₃ N₄ bodies currently produced in accordancewith the reaction bonding method are merely somewhat greater than 80%.This insufficient density impedes to improve the strength of sinteredbodies. Moreover, an extremely long time period is required for thenitridation. For example, the reaction treatment requires at least twodays in shorter cases and, in some longer cases, takes as long as 10days or more. This is certainly a great problem in adopting the reactionbonding method.

The pressureless sintering method is a method to sinter a siliconnitride green compact which has in advance been formed into a desiredshape. Thus, this method is rather easy to produce sintered products ofcomplex configurations. However, the densities of such products arelimited and are around 96% even for higher ones.

As an improved method over the above-described pressureless sinteringmethod, there has been proposed to sinter a green compact, which hasbeen preformed into a desired shape, in an N₂ gas atmosphere of severalatm to several ten atm as described in Japanese Patent Laid-open No.47015/1977 laid-open Apr. 14, 1977 and naming as an inventor MamoruMitomo as well as Japanese Patent Laid-open No. 102320/1978 laid-openSep. 6, 1978 and naming as an applicant General Electric Company. Suchan improved method can provide sintered bodies having a density as highas 98% at maximum. However, it still fails to meet both of therequirements, namely, requirements for the formation of complicatedshapes and the high densification. Especially, as a serious obstacle tothe high densification, there is mentioned the thermal decompositionproblem of Si₃ N₄ upon sintering. Namely, upon sintering Si₃ N₄, MgO,SiO₂, Al₂ O₃ and/or the like are incorporated as a sintering aid. Thesecompounds are however believed to volatilize during the sintering stepby their reaction with Si₃ N₄ as described below.

    Si.sub.3 N.sub.4 +3MgO→3SiO↑+3Mg↑+2N→.sub.2

    Si.sub.3 N.sub.4 +3SiO.sub.2 →6SiO↑+2N↑.sub.2

    Si.sub.3 N.sub.4 +Al.sub.2 O.sub.3 →2AlN+3SiO↑+N↑.sub.2

On the other hand, Si₃ N₄ per se is known to undergo a thermaldecomposition as described below.

    Si.sub.3 N.sub.4 →3Si+2N↑.sub.2

Due to these thermal decomposition reactions, a weight loss normallytakes place during a sintering step and, in some instance, such a weightloss exceeds the rate of density increase due to the shrinkage of asintered body upon sintering. It has been reported that such a weightreduction may, in some instances, reach as high as 50%. Theabove-described sintering method in an N₂ gas atmosphere was proposed asa measure for inhibiting the thermal decomposition but is still believedto cause a weight loss of several percents.

According to a research carried out by the present inventors, it hasbeen recognized that the weight loss caused by the thermal decompositionhas a close relationship with the density of a green compact beforesintering as will be described later. The present inventors have alsofound that the weight loss due to thermal decomposition tends to becomeconsiderable where the initial density (i.e., the density of a greencompact before sintering) is low but it becomes smaller as the initialdensity increases. From this finding, the decomposition reaction isconsidered to proceed, in the conventional sintering method in an N₂ gasatmosphere, to the interior of a sintered body since the density of agreen compact before sintering is merely 60% or so and its pores arecompletely open, and reaction products of the thermal decomposition arethus allowed to diffuse from the interior of the sintered body to theexterior of the same.

It is certainly an effective measure to increase the nitrogen gaspartial pressure to suppress the aforementioned thermal decomposition inthe sintering method in an N₂ gas atmosphere. However, an N₂ gas partialpressure of several ten atm or so, which has been employed in theconventional method, is by no means sufficient to effectively prohibitthe thermal decomposition from the thermodynamical viewpoint.

As a method for highly densifying Si₃ N₄, it has been known to employ anHIP treatment using Ar gas. To follow this method, Si₃ N₄ powder isgenerally sealed hermetically in a capsule made of a gas-impermeablematerial such as glass and then subjected to an HIP treatment. Thismethod is however impractical to produce a sintered body of a complexshape as it is extremely difficult to shape a capsule corresponding withthe configurations of the intended product. It is also impractical as itstill involves many problems to be solved, such as filling uniformly Si₃N₄ powder in capsules, measures to avoid the reaction between Si₃ N₄ andcapsules, and measures to decapsulate resultant Si₃ N₄ bodies from thecapsules. In addition, it has also been proposed to close the pores of agreen compact to a certain extent prior to applying an HIP treatment bysubjecting the green compact to a presintering treatment. It is howeverdifficult, generally speaking, to achieve a high density by this methodas a considerable weight loss takes place due to the thermaldecomposition of Si₃ N₄.

On the other hand, a large amount of research effort has been directedto the discovery of sintering aids suitable for producing high densitysintered Si₃ N₄. However, under the present circumstances, it is stillfar away from the goal. It is considered to be promising to employ Y₂ O₃or a mixture of Y₂ O₃ and Al₂ O₃ as a sintering aid for Si₃ N₄. As amatter of fact, it has been known that a sintered body having excellentmechanical properties can be obtained by mixing a suitable amount ofsuch a sintering aid and Si₃ N₄ powder and by sintering the resultantmixture in accordance with the hot pressing method. However, such Y₂ O₃system sintering aids are accompanied by the drawback that thepressureless sintering method, which is rather easy to produce bodies ofcomplex configurations, cannot be applied to obtain sintered bodies ofhigh density and strength. Thus, such Y₂ O₃ system sintering aids areemployed exclusively for the production of bodies of simpleconfigurations in accordance with the hot pressing method.

As a result of extensive research conducted by the present inventors, ithas been unexpectedly found that high density sintered siliconnitride(Si₃ N₄) of a relative density of 98% or higher may be obtainedby compacting silicon nitride powder into a green compact having adesired shape, presintering the green compact to a relative density ofat least 92%, and then subjecting said presintered body to a hotisostatic pressing in an inert gas atmosphere of a temperature in therange of 1500°-2100° C. and of a nitrogen gas partial pressure of atleast 500 atm until the former relative density is reached. In thisinvention, the gas used in the hot isostatic pressing is only thenitrogen or a gas mixture including the nitrogen. This method may besimilar to the aforementioned prior art methods in an incorporation ofthe HIP method. However, the above finding of the present inventors isfundamentally different from the conventional HIP method as, in theformer case, the relative density of a presintered body is raised to 92%or higher and it is then subjected to an HIP treatment in an inertatmosphere of the above specific temperature and N₂ gas partialpressure.

The present inventors have also found that an incorporation of a Y₂ O₃-Al₂ O₃ -MgO system mixture as a sintering aid in Si₃ N₄ powder canprovide under normal pressure a presintered body of a higher densitycompared with the addition of a conventional Y₂ O₃ or Y₂ O₃ -Al₂ O₃system sintering aid.

As a result of an experiment carried out by the present inventors todetermine Whether there is any relationship between the relative densityof sintered Si₃ N₄ and its strength, it has been found that they are notalways corelated to each other. The inventors expanded the study andcarried out a further experiment while paying attention to β-Si₃ N₄ inpresintered bodies. As a result of the further experiment, it has beenfound that the strength of a sintered Si₃ N₄ body corelates to thecontent of β-Si₃ N₄ in the presintered Si₃ N₄.

Furthermore, it has been realized that the present sintering method,which makes use of an HIP treatment, involves the following problem inorder to more effectively apply the same to the industry. Namely, apresintered body is discharged from a presintering furnace uponcompletion of the presintering step and cooled prior to charging thesame into an HIP furnace. The thus-presintered body is then charged at acooled temperature into the HIP furnace, although the HIP treatmentrequires 1000° C. or higher, particularly, a high temperature of atleast 1500° C. for Si₃ N₄.

Such a cooling of a presintered body may be unavoidable where thesintering method is performed batch by batch. This however leadsundoubtedly to a considerable loss of heat energy in view of the factthat the presintered body has been heated to a considerable extentduring its presintering step. In addition, when a presintered body whichhas been presintered at high temperatures is rapidly cooled at thesurface thereof or, after the rapid cooling, when rapidly heated,extremely fine cracks and fissures are likely to occur, wherebyunavoidably deteriorating its strength and causing other problems withrespect to its quality.

The present inventors have also found a measure capable of effectivelyovercoming the above problems of the thermal energy loss and strengthdeterioration caused by the rapid cooling of presintered and sinteredbodies. Namely, it has been discovered that the thermal energy loss maybe reduced and a high density sintered Si₃ N₄ body can be obtained bycharging a presintered body, while maintaining its temperature above500° C., into an HIP furnace which has been in advance heated at 500° C.or higher to conduct the HIP treatment, discharging the thus-sinteredSi₃ N₄ body from the HIP furnace at a temperature of at least 500° C.,and then subjecting the same to a heat treatment at 500° C. or higher ina heat treatment furnace.

SUMMARY OF THE INVENTION

According to one aspect of this invention, there is provided a methodfor producing high density sintered silicon nitride(Si₃ N₄) having arelative density of at least 98%. The method comprises forming siliconnitride powder into a desired shape to obtain a silicon nitride greencompact, presintering said green compact to a presintered body having arelative density of at least 92%, and then subjecting said presinteredbody to a hot isostatic pressing in an inert gas atmosphere of atemperature in the range of 1500°-2100° C. and of a nitrogen gas partialpressure of at least 500 atm until the former relative density isreached.

In another aspect of this invention, the silicon nitride powder maycontain a sintering aid, for example, 3-13% by weight of Y₂ O₃, 0.5-4%by weight of Al₂ O₃ and 0.5-6% by weight of MgO.

In a further aspect of this invention, the presintered body contains20-80% by weight of β-Si₃ N₄ and the content of β-Si₃ N₄ in the sinteredsilicon nitride(Si₃ N₄) is increased to 80% or higher through the hotisostatic pressing.

In a still further aspect of this invention, the presintered body isdischarged at a temperature of at least 500° C. from a presinteringfurnace and immediately charged at a temperature of 500° C. or higherinto a hot isostatic pressing furnace which has been preheated to 500°C. or higher in advance; upon completion of the hot isostatic pressing,the sintered body is discharged at a temperature of at least 500° C.from the hot isostatic pressing furnace and then charged at atemperature of 500° C. or higher into a heat treatment furnace, whichhas been maintained at 500° C. or higher, to conduct a heat treatment ina non-oxidizing gas atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic illustration of an example of the relationshipbetween presintering temperature and the content of α-Si₃ N₄ inresultant presintered body;

FIG. 2 is a graphical representation of the content of α-Si₃ N₄ inpresintered Si₃ N₄ body at various sintering time periods;

FIG. 3 is a graphical illustration showing changes in relative densityof various presintered Si₃ N₄ bodies before and after their hotisostatic pressing treatments;

FIG. 4 shows diagrammatically the relationship between the relativedensities of various presintered Si₃ N₄ bodies and the percentage weightvariations of their corresponding sintered Si₃ N₄ bodies obtainedthrough a hot isostatic pressing treatment;

FIG. 5 is a graphical representation of the relative densities ofvarious presintered bodies obtained in accordance with the presentinvention and their presintering temperatures; and

FIG. 6 is a diagrammatic illustration showing the relationship betweenthe presintering time periods of the presintered Si₃ N₄ bodies and theirrelative densities.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

First of all, the raw material Si₃ N₄ employed in the method of thisinvention may be obtained through the nitridation of metallic Si or maybe prepared from SiO₂ by its reduction or from SiCl₄ or Si(NH)₂ inaccordance with the gas phase reaction method or thermal decompositionmethod. In view of the bend strength of sintered Si₃ N₄ to be obtained,it is preferable to employ that prepared in accordance with the gasphase reaction method or the thermal decomposition method. In addition,the ratio of both amorphous forms in a raw material Si₃ N₄ powder, inother words, the ratio of α-Si₃ N₄ to β-Si₃ N₄ is desirously such thatthe content of β-Si₃ N₄ in a presintered body to be obtained through itspresintering would range 20-80% by weight as will be described later inthis specification. It is particularly preferred that the raw materialSi₃ N₄ contains α-Si₃ N₄ in a proportion of at least 80% by weight.

As a sintering aid which may be incorporated in the aforementioned Si₃N₄ powder, there may be mentioned the oxide or nitride of Y, Al, Mg, Tior the like, or ZrO₂, BeO, La₂ O₃, CeO₂ or the like. Such compounds maybe used solely or in combination. A sintering aid consisting of a Y₂ O₃-Al₂ O₃ -MgO system mixture is most effective. Such a sintering aid maybe incorporated in the powder up to 30% by total weight. It is howeverparticularly preferable to add Y₂ O₃, Al₂ O₃ and MgO respectively inamounts of 3-13% 0.5-4% and 0.5-6%, all by weight. The forming of Si₃ N₄powder containing such a sintering aid may be effected, depending on theshape to be formed, by injection molding, extrusion, die pressing orisostatic pressing method.

The green compact formed into a predetermined shape as mentioned aboveis then presintered to obtain a presintered Si₃ N₄ body whose relativedensity is 92% or higher and, preferably, which contains 20-80% byweight of β-Si₃ N₄. The presintering may be carried out in accordancewith the conventionally well-known hot pressing method or a sinteringmethod under atmospheric pressure or so or in an atmosphere of highpressure N₂ gas. It is however preferable to presinter in anon-oxidizing atmosphere such as N₂ gas or the like. The presinteringtemperature may vary depending on the type of sintering aid to beemployed, its content and sintering time but a temperature range of1400°-1800° C. is generally suitable. FIG. 1 is a diagrammaticillustration of an example of the relationship between presinteringtemperature and the content of α-Si₃ N₄ in resultant presintered body.It shows the relationship between presintering temperatures and thecontents of α-Si₃ N₄ in the corresponding presintered bodies when Si₃ N₄powder was added with 6% by weight of Y₂ O₃ and 2% by weight of Al₂ O₃as well as, respectively, 1%, 3% and 5% by weight of MgO as a sinteringaid and then presintered for 200 minutes by varying the presinteringtemperature. As apparent from FIG. 1, the content of α-Si₃ N₄ in thepresintered body varies in accordance with the amount of the sinteringaid to be incorporated The content of α-Si₃ N₄ ranges about 80%-20% byweight for a presintering temperature range of 1500°-1600° C. Therefore,to limit the content of β-Si₃ N₄ in a presintered body within a range of20%-80% by weight, it is preferable to employ as the presinteringtemperature a lower temperature range of 1500°-1600° C.

On the other hand, the presintering time may also change depending onthe type and content of sintering aid to be used as well as presinteringtemperature. A period of 50-200 minutes is generally suitable.

FIG. 2 illustrates an example of the relationship between presinteringtime and the content of α-Si₃ N₄ in a presintered body. It illustratesthe relationship between presintering time periods and the contents ofα-Si₃ N₄ in resultant presintered bodies when Si₃ N₄ powder was addedwith 6% by weight of Y₂ O₃ and 2% by weight of Al₂ O₃ as well as,respectively, 1%, 3% and 5% by weight of MgO as a sintering aid and thenpresintered at 1600° C. by changing the presintering time. As readilyenvisaged from FIG. 2, the content of α-Si₃ N₄ in a presintered bodyvaries depending on the amount of the sintering aid to be added. For asintering time period of 50-200 minutes, the content of α-Si₃ N₄ in apresintered body ranges approximately 80%-20%. Thus, to limit thecontent of β-Si₃ N₄ in a presintered body within 20%-80%, it is easilygathered that the sintering time should preferably be from 50 minutes to200 minutes.

It is important to obtain through the presintering treatment apresintered body whose bend strength can be increased in an HIPtreatment, which will be described later. It is desirous that apresintered body contains β-Si₃ N₄ in an amount of 20%-80% by weight.Although reasons for the importance of β-Si₃ N₄ content in a presinteredbody have not yet been known, it appears that the presence of β-Si₃ N₄in a proportion of 80% or less in other words, the presence of α-Si₃ N₄in an amount of 20% or more is effective to prevent β-type crystals fromcoarsening during their formation or growth through an HIP treatment inview of the fact that an observation of fracture surfaces of a sinteredSi₃ N₄ body obtained by an HIP treatment, said fracture surfaces havingbeen formed through its bend strength test, indicated coarsenedneedle-like crystals where the content of β-Si₃ N₄ in a presintered bodyexceeds 80%.

The thus-obtained presintered body is then subjected to an HIP treatmentto prepare a high density sintered Si₃ N₄ whose relative density is atleast 98%, and preferably, whose β-Si₃ N₄ content is 80% by weight ormore, more preferably, 90% or higher. Here, although the relativedensity of a presintered body is defined to be at least 80%, somepresintered bodies whose relative densities are rather close to thelower limit may still contain open pores, in other words, poresextending from the interior to the surface and opening into theexterior. If such presintered bodies are subjected to an HIP treatmentas they are, the HIP treatment can eliminate closed pores but open poreswould still remain. Thus, it is rather difficult to obtain a sinteredbody whose relative density is 98% or higher. It is thus preferred toapply a pore-closing treatment to presintered bodies whose relativedensities are low by coating thereon the oxide or nitride of Si, Al, orthe like prior to subjecting them to an HIP treatment. According to astudy of the present inventors, pores in a presintered body do not openinto its surface but substantially closed where its relative density is92% or higher. Accordingly, such a presintered body can be subjecteddirectly to an HIP treatment without need for any pore-closingtreatment. Even if the relative density is 92% or higher, thepresintered body may obviously be subjected to a pore-closing treatmentprior to conduct an HIP treatment thereon, for example, for the purposeof avoiding the decomposition reaction of Si₃ N₄ where Ar gas is used asa pressure medium gas.

The HIP treatment is carried out in an atmosphere of an inert gas suchas Ar gas, N₂ gas, or the like. It is however preferable to conduct theHIP treatment in an N₂ gas atmosphere for the reasons that N₂ gas canprevent the decomposition reaction of Si₃ N₄ and thus highly densify thesame.

For HIP treatment, a temperature range of 1500°-2100° C., morepreferably, 1700°-2000° C. may be employed. The HIP temperature isdesirously higher than that employed in the presintering step. Needlessto say, the HIP temperature must be lower than the decompositiontemperature of Si₃ N₄. Although the decomposition temperature increasesas the HIP pressure goes higher, it is preferable to carry out an HIPtreatment at a temperature lower by at least 100° C., than thedecomposition temperature of Si₃ N₄ at the pressure of the HIPtreatment. If the HIP temperature is below 1500° C. the strength of asintered body can be improved only by 20-30% in comparison with thestrength of the presintered body before the HIP treatment although thepresintered body can be densified to its theoretical density or so. Onthe other hand, above 1500° C., Si₃ N₄ undergoes not only densificationbut also considerable β-transformation, whereby remarkably improving itsstrength. Furthermore, an HIP treatment at higher temperatures causesthe coarsening of grains. An abrupt strength reduction takes place above2100° C. As the HIP temperature becomes higher, the HIP apparatus has tobe made greater and its thermocouples are deteriorated to a greaterextent. Thus, a suitable HIP temperature range is, as described above,1500°-2100° C.

Next, the HIP pressure is preferably 500 atm or higher. Below 500 atm,the HIP treatment requires long time. Furthermore, the extent of thedecomposition reaction of Si₃ N₄ would become greater in proportion withthe HIP time, thereby leading to a weight reduction and making itdifficult to achieve the high densification. Therefore, it is desirousto set the HIP pressure at at least 500 atm, preferably, above 700 atm.Where N₂ gas is employed as a constituent of the atmosphere for HIPtreatment, the N₂ partial pressure is preferably at least 500 atm.

As the HIP pressure increases, the decomposition reaction of Si₃ N₄would be more effectively suppressed and it would become easier toattain the high densification. However, excessively high pressures areimpractical for HIP treatment as more time is required to reach suchhigh pressures and larger HIP treatment system is also required, led bythe compressor for raising the pressure and including the main casing,i.e., pressure vessel. Thus, from the practical point of view, it isdesirous to carry out the HIP treatment at pressures up to 2500 atm.

On the other hand, the HIP treatment time may preferably range from 1minutes to 3 hours and is generally about 30 minutes or so.

It is possible to crystallize partially or completely the glassy phasesat grain boundaries of Si₃ N₄ in the course of the HIP treatment,thereby improving its strength further.

As has been described above, a presintered Si₃ N₄ body can be convertedto a high density sintered Si₃ N₄ body having a relative density of atleast 98% through an HIP treatment.

When an HIP treatment is applied while burying a presintered body inpowder consisting principally of at least one nitride ceramics selectedfrom silicon nitride, aluminum nitride and boron nitride, thedecomposition reaction of the presintered body can be suppressed and ahighly strong and dense sintered silicon nitride can be provided.

Where the content of β-Si₃ N₄ in a presintered body is controlled withina range of 20-80% its sintering can be performed easily through an HIPtreatment, thereby achieving easily high densification. At the sametime, in the thus-obtained sintered Si₃ N₄ body, most Si₃ N₄ have beentransformed into β-Si₃ N₄ and its structure is very fine, thus producinghigh density sintered Si₃ N₄ with an improved bend strength.

Furthermore, the HIP treatment not only facilitates high densificationbut also enables a high temperature treatment of Si₃ N₄. Thus, it ispossible to shorten the time required to transform from α-Si₃ N₄ toβ-Si₃ N₄, leading to a cut-down of its production cost. Where a mixtureof Si₃ N₄ powder and a sintering aid is compacted and presintered, theforming is rather easy even if it has a complex shape. As mentionedabove, the HIP treatment enables to achieve high densification andimproved strength. Therefore, the present invention has an advantagethat sintered Si₃ N₄ of a desired complex shape and having high densityand strength can be produced easily. Consequently, the method of thisinvention is extremely practical as an industrial production method ofsintered Si₃ N₄ bodies.

Now, a sintering method according to another embodiment of thisinvention, which method is capable of reducing the thermal energy lossand minimizing the deterioration in strength due rapid temperaturechanges, is sequentially described in the order of its steps. Thissintering method is carried out in three steps, namely, presinteringstep, HIP treatment step, and heat treatment step. Of the three steps,the presintering and HIP treatment steps are carried out as describedbefore.

As mentioned above, where a presintered body has a relative density of92% or higher, its surface is substantially free of pores and pores inthe body do not extend to the surface. Thus, a high densification iseasily feasible by the above-described HIP treatment. Taking the bendstrength of sintered Si₃ N₄ into consideration, it is suitable to makethe content of α-Si₃ N₄ in the raw material powder be at least 80%.

The thus-presintered Si₃ N₄ has now gotten through the first step and isthen charged, without allowing its temperature to cool down to roomtemperature and while maintaining a temperature of 500° C. or higher,into an HIP furnace for subjecting the same to an HIP treatment.

For carrying out the HIP treatment at high temperatures and pressures,the HIP is usually preheated to a high temperature of 500° C. or higherand maintained at that temperature. Thus, it is possible to design theproduction facilities so as to charge the presintered Si₃ N₄continuously into the HIP furnace.

By charging the presintered Si₃ N₄ into the HIP furnace without oncelowering its temperature and conducting the HIP treatment, it ispossible to save thermal energy which is required to raise thetemperature of the presintered Si₃ N₄ in the above-mentioned sinteringmethod making use of an HIP treatment, as the presintered Si₃ N₄ is oncewithdrawn from a presintering furnace, allowed to cool down, and thencharged into the HIP furnace in the ordinary two-step sintering methodwhich combines a presintering step with an HIP treatment. It is alsopossible to shorten the HIP treatment cycle. Furthermore, it bringsabout another advantage that the occurrence of cracks due to rapidcooling upon withdrawal of the presintered Si₃ N₄ and rapid heating uponcharging into the HIP furnace can be effectively prevented.

The HIP treatment can be carried out directly on a presintered body in aknown HIP furnace without need for any capsule. It is however necessaryto conduct the HIP treatment in an N₂ gas atmosphere as mentioned aboveso as to prevent the decomposition reaction of Si₃ N₄ and achieve a highdensification.

A sintered body, which has been densified through the above HIPtreatment, is then discharged at a temperature of at least 500° C. fromthe HIP furnace and then subjected to a heat treatment in anon-oxidizing gas atmosphere by charging the same into a heat treatmentfurnace maintained at 500° C. or higher.

Conventionally, a sintered body, which has been subjected to an HIPtreatment, was cooled to room temperature after the HIP treatment toobtain a final product. It is possible to enhance the strength of asintered Si₃ N₄ body and to improve its quality further by employing theheat treatment. It is important to conduct this heat treatment in anon-oxidizing atmosphere such as N₂ gas or the like to avoid the thermaldecomposition of Si₃ N₄.

The heat treatment temperature varies, similar to the HIP treatmenttemperature, depending on the type and content of the sintering aid aswell as HIP conditions. A temperature of 500° C. or higher is consideredto be effective for increasing the strength of a sintered body. The heattreatment does not require too long time but 5-10 minutes aresufficient.

Sintered Si₃ N₄ is produced through the first, second and third steps asdescribed above. Since the resultant sintered body is not subjected torapid cooling or heating in the course of its production, the occurrenceof cracks is prevented, thereby resulting in excellent quality havinghigh strength and density. In addition, the strength of sintered Si₃ N₄can be enhanced further by crystallization of glassy phrases at grainboundaries therein through the heat treatment. Moreover, also uponconducting this heat treatment, the sintered body is discharged at 500°C. or higher after the HIP treatment and then charged into the heattreatment furnace which is also maintained at 500° C. or higher. Thus,it is possible to minimize the thermal energy loss which otherwise takesplace in the heat treatment step.

As has been described above, Si₃ N₄ is presintered at high temperatures,preferably, in the range of 1000°-1800° C., charged without allowing itto cool down to room, temperature and while maintaining it above 500° C.into an HIP furnace also preheated to 500° C. or higher to perform adensification treatment through the HIP treatment, and then charging thesintered body after the HIP treatment, while maintaining it above 500°C., into a subsequent heat treatment furnace which has beforehand beenmaintained above 500° C. This can prohibit the occurrence of cracks dueto thermal shocks during rapid cooling or heating, thereby easilyproviding sintered Si₃ N₄ of high strength and density. By strictlycontrolling the treatment temperatures throughout the presintering, HIPtreatment and heat treatment steps, it is possible to realize the desirefor the continuation of these steps and reduce the loss of thermalenergy to be consumed. Thus, the above sintering method is in conformitywith the trend of energy saving which is of a primary concern these daysand realizes to shorten the production cycle and to improve theefficiency of production. Therefore, this sintering method is expectedto enjoy its commercial utility as an economical and practicalproduction process.

The invention will hereinafter be described in further detail inaccordance with the following varied examples.

EXAMPLE 1

To commercially available Si₃ N₄ obtained through the nitridation of Sipowder, were added as a sintering aid 6% by weight of Y₂ O₃ powder and2% by weight of Al₂ O₃ respectively. The resulting mixture was ballmilled for 10 hours in an organic solvent and then dried to powder.Subsequent to cold compacting the powder under a pressure of 1 ton/cm²,it was formed into presintered bodies having various relative densitiesin accordance with the sintering method in an N₂ gas atmosphere or thehot pressing sintering method.

The thus-obtained presintered bodies of various densities were subjectedto an HIP treatment for 1 hour in an Ar gas or N₂ gas atmosphere of1800° C. and 1000 atm. A measurement of density changes before and afterthe HIP treatment gave a result as shown in FIG. 3.

As apparent from FIG. 3, when the HIP method using N₂ gas(hereinafterreferred to as "the N₂ HIP method) is compared with the HIP methodemploying Ar gas(hereinafter referred to as "the Ar HIP method"), it isappreciated that, in the bodies presintered by the Ar HIP method, thedensities after the HIP treatment remain below their correspondingdensities before the HIP treatment over the entire range of the relativedensities and the weight loss due to the thermal decomposition of Si₃ N₄is thus greater in ratio than the densification effect owing to the HIPtreatment. On the other hand, it is also envisaged from the figure that,in the bodies presintered in accordance with the N₂ HIP method, nosubstantial differences are observed in relative density between beforeand after the HIP treatment in a range of the relative density beforethe HIP treatment of up to 90% or so. This indicates that, up to therelative density of 90% or so, more inner pores communicate to thesurface and are present as open pores and thus the HIP effect can bescarecely expected. However, as soon as the relative density of apresintered body exceeds 91% or so, its density after the HIP treatmentincreases abruptly, and, beyond 92%, high density sintered bodies of arelative density of at least 98% can be obtained always.

From the above example, it has been proven that a high density sinteredbody having a relative density of at least 98% can be obtained by the N₂HIP method where the density of its corresponding presintered body is92% or so before the HIP treatment. It has also become clear that the ArHIP method is totally hopeless in achieving such a high density.

EXAMPLE 2

The procedure of Example 1 was followed to obtain presintered bodies ofvarious relative densities. They were subjected to an HIP treatment forone hour and at a temperature of 1700° C. or 1900° C. using N₂ gas or Argas. A measurement of the weight loss of each body after the HIPtreatment gave a result as illustrated in FIG. 4. As appreciated fromthis figure, the weight loss due to the HIP treatment was small in theN₂ HIP treatment and was always limited below 0.5% under the HIPconditions, particularly, where the relative density was 92% or higher.Even a tendency of weight increase is observed in the higher range ofthe relative density before the HIP treatment. This weight gain appearsto attribute to the nitridation reaction of fee Si in starting powder.On the other hand, in the HIP method, as the temperature increases thedecomposition reaction tends to proceed further. Thus, a considerableweight loss is observed.

In view of the above finding, it is appreciated that the N₂ HIP methodaccording to this invention can render a sintered body highly dense andat the same time can improve the quality of the final product since thedecomposition reaction of Si₃ N₄, which is liable to decompose, can beminimized even in a high temperature range and, where Si₃ N₄ powderobtained through the nitridation of Si powder is used, even a phenomenonof weight gain is observed owing to the nitridation of unreacted Si.

EXAMPLE 3

The powder obtained in Example 1 was cold compacted under a pressure of1 ton/cm², followed by its presintering at 1900° C. in an N² gasatmosphere of 30 atm and for one hour, resulting in the preparation of apresintered body having a relative density of 95%. The thus-preparedpresintered bodies are respectively subjected to the N₂ HIP treatmentand Ar HIP treatment under HIP conditions of 1900° C.×700 atm×20minutes. Physical properties of the presintered and sintered bodies weremeasured respectively before and after the HIP treatments. Results aresummarized in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Bend strength    Weight                                               Relative                                                                              Three-point loading                                                                            loss                                                 density test (Kg/mm.sup.2)                                                                             after HIP                                            (%)     Room temp. 1200° C.                                                                         (%)                                       ______________________________________                                        Presintered                                                                            95        71         38      --                                      body                                                                          Ar HIP sin-                                                                            96        76         41      5.6                                     tered body                                                                    N.sub.2 HIP sin-                                                                       100       96         58      1.1                                     tered body                                                                    ______________________________________                                    

As apparent from the above table, the N₂ HIP sintered body of thisinvention is densified to the true density and its mechanical strengthis considerably improved while suppressing the weight loss due to theHIP treatment relatively low. On the other hand, with the Ar HIPsintered body; an improvement to the density and mechanical propertiescannot be expected so much and the weight loss due to the HIP treatmentreached a considerably high value.

EXAMPLE 4

To commercially available Si₃ N₄ powder obtained by nitridation of Sipowder, was added 5% by weight of MgO powder as a sintering aid. Theresultant mixture was ball milled in an organic solvent for 10 hours andthen dried. The thus-obtained powder was then cold compacted under apressure of 1 ton/cm². Then, the resulting green compact was presinteredin accordance with the N₂ gas atmosphere sintering method, therebyproducing a presintered body having a relative density of 93%. Thethus-prepared presintered bodies were buried in various ceramics powderand subjected to the N₂ HIP treatment under the conditions of 1750°C.×1000 atm×0.5 hour. The density, weight change and strength of each ofthe resultant N₂ HIP sintered bodies were measured. Measurement resultsare given in Table 2.

                  TABLE 2                                                         ______________________________________                                                                       Bend strength                                  Type of      Relative  Weight  Three-point load-                              ceramics     density   loss    ing test (room                                 powder       (%)       (%)     temp. Kg/mm.sup.2)                             ______________________________________                                        Not used     98        2       63                                             50% Si.sub.3 N.sub.4                                                                       100       0.3     75                                             + 50% B.sub.4 C                                                               30% Si.sub.3 N.sub.4                                                                       100       0.5     72                                             + 70% BN                                                                      70% BN       100       0.2     78                                             + 30% Al.sub.2 O.sub.3                                                        80% AlN + 15% BN                                                                           99        0.9     69                                             + 5% SiO.sub.2                                                                ______________________________________                                    

As envisaged from the above table, those subjected to an HIP treatmentwhile being buried in ceramics powder are superior in every aspects,namely, density, strength and weight reduction to that HIP treatedwithout being buried in any ceramics powder. This advantage appears tohave derived from the fact that the ceramics powder was decomposedduring the HIP treatment and increased the partial pressures ofdecompositon products in the circumference of the presintered body,thereby inhibiting the decomposition reaction of the presintered body asthe ceramics powder employed in the above example were each powdercontaining a nitride as its main component. Therefore, it is extremelyeffective to conduct an HIP treatment while burying a presintered bodyin ceramics powder as a method for suppressing the thermal decompositionof Si₃ N₄ during the HIP treatment of a presintered Si₃ N₄ body andobtaining sintered Si₃ N₄ of high strength and density. Here, theceramics powder to be employed must be powder of a nitride in order toincrease the partial pressures of the decomposition products in thevicinity of the presintered body.

EXAMPLE 5

To two types of Si₃ N₄ powder, one being commercially available Si₃ N₄powder obtained through the nitridation of Si powder(i.e., by theSi-nitridation method) and the other being Si₃ N₄ powder produced bythermally decomposing a reaction product of silicon tetrachloride andammonia(SiCl₄ +NH₃ synthesis method), was added as a sintering aid 6% ofY₂ O₃ powder. The resultant mixtures were mixed in an organic solventfor ten hours by means of a ball mill and then dried, Each of theresulting powder was cold compacted under a pressure of 1 ton/cm²sintered by the hot pressing sintering method to presintered bodieshaving a relative density of 92%, They were then subjected to an N₂ HIPtreatment under the conditions of 1900° C.×800 atm×0.5 hour. Physicalproperties of resultant HIP sintered bodies are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                       Bend strength                                  Type of     Relative  Weight   Three-point load-                              Si.sub.3 N.sub.4                                                                          density   loss     ing test (room                                 Powder      (%)       (%)      temp. Kg/mm.sup.2)                             ______________________________________                                        Si-nitridation                                                                            98        2.5      79                                             method                                                                        SiCl.sub.4 + NH.sub.3                                                                     100       0.1      103                                            synthesis method                                                              ______________________________________                                    

From the above table, it is appreciated that, compared with Si₃ N₄powder obtained in accordance with the Si-nitridation method, the Si₃ N₄produced by the SiCl₄ +NH₃ synthesis method is densified higher and thusmade stronger as well as its weight loss is considerably low. Thisdifference is considered to stem from the fact that Si₃ N₄ powderobtained by the synthesis method contains less impurities and itsparticle size is very small compared with that resulted from theSi-nitridation method, the opening ratio of its inner pores is thus verysmall although both pre-sintered bodies had the same relative density,the densifying effect of the HIP treatment is high and the progress ofthe thermal decomposition can be decelerated to a considerable extent.

EXAMPLE 6

Y₂ O₃, Al₂ O₃ and MgO powder were added in various proportions tocommercially available Si₃ N₄ powder. Resulting powder mixtures werepress-formed under a pressure of 1000 Kg/cm² to form green compacts,which were subsequently subjected to a pressureless sintering at atemperature of 1550°-1780° C. in an atmosphere of nitrogen gas. Table 4shows the proportion of each powder, sintering conditions and therelative density and strength of each of the sintered bodies obtained.

                                      TABLE 4                                     __________________________________________________________________________        Composition   Sintering   Relative                                                                           Bend strength                              Sample                                                                            (wt %)        conditions  density                                                                            at room temp.                              No. Si.sub.3 N.sub.4                                                                  Y.sub.2 O.sub.3                                                                  Al.sub.2 O.sub.3                                                                  MgO                                                                              Temp (°C.)                                                                   Time (min)                                                                          (%)  (Kg/mm.sup.2)                                                                        Note                                __________________________________________________________________________    1   92  6  2   -- 1600  200   79   --     Comparative                                                                   example                             2   95.8                                                                              -- 3.0 1.2                                                                              1750  180   91   31.0   Comparative                                                                   example                             3   91  -- 4.5 4.5                                                                              1780  180   92   38.5   Comparative                                                                   example                             4   91  6  2   1  1600  200   86   32     This                                                                          invention                           5   89  6  2   3  1600  200   93   57     This                                                                          invention                           6   87  6  2   5  1600  200   95   70     This                                                                          invention                           7   91  6  2   1  1550  200   91   44     This                                                                          invention                           8   89  6  2   3  1550  200   96   62     This                                                                          invention                           9   87  6  2   5  1550  200   96   74     This                                                                          invention                           __________________________________________________________________________

Now, a discussion is extended on each result shown in the above table.In Sample No. 1 using a conventional Y₂ O₃ -Al₂ O₃ system sintering aid,the sintering did not proceed to a sufficient degree because it wasconducted under normal pressure. Thus, it was unable to measure its bendstrength. In Sample Nos. 2 and 3 containing no Y₂ O₃, despite of theemployment of high sintering temperatures, their bend strengths remainedat the level of 30 Kg/mm² and were still far below the practicallymeaningful strength. On the other hands, in the examples making use of aY₂ O₃ -Al₂ O₃ -MgO system sintering aid, it is possible to obtain a bendstrength of the order of 40 Kg/mm² or higher provided that the sinteringconditions are suitably selected. Particularly, in Sample Nos. 6 and 9,high strength sintered bodies of 70 Kg/mm² or higher were obtained.

As apparent from these facts, use of a Y₂ O₃ -Al₂ O₃ -MgO systemsintering aid permits the adoption of the pressureless sintering methodwhich has been considered impractical for conventional Y₂ O₃ -Al₂ O₃system sintering aids. Accordingly, the raw material powder can beformed into a desired complex shape in accordance with the injectionmolding, extrusion or slip casting method. This assures that sinteredproducts of various complex shapes can be prepared easily at a lowproduction cost.

Y₂ O₃, Al₂ O₃ and MgO are fused together at a sintering temperature andsinter Si₃ N₄ into a dense mass. However, if Y₂ O₃ is added in aproportion exceeding 13% by weight, the oxidation resistance property ofthe sintered body will be lessened sharply. Thus, it is desirous tolimit the proportion of Y₂ O₃ below 13%. On the other hand, if thecontent of Y₂ O₃ is too low, the sintering will become rather difficult.Thus, it is desirous to add Y₂ O₃ in a proportion of at least 3%. Al₂ O₃acts to reduce the deterioration of the oxidation resistance property ofa sintered product due to the incorporation of Y₂ O₃, especially, theabrupt lowering of the oxidation resistance property above 1000° C.However, an excessive incorporation of Al₂ O₃ will lead to adeteriorated strength of a sintered body. Thus, it is desirous to limitthe content of Al₂ O₃ below 4%. On the other hand, if added too little,the above inhibitory action against the deterioration of the oxidationresistance property cannot be expected. Therefore, it is preferable toadd at least 0.5% of Al₂ O₃. Next, MgO is a sintering aid whichconstitutes one of features of this invention. It is a component whichenables the adoption of the pressureless sintering method. Anincorporation of less than 0.5% of MgO does not substantially exhibitits densifying effect in the pressureless sintering. Accordingly, it isdesirous to add MgO in an amount of at least 0.5%, preferably, 1.5% ormore. However, if MgO is added too much, there will be a danger oflowering the strength. Thus, it is also desirous to set its upper limitat 6%.

Regarding the presintering temperature when a Y₂ O₃ -Al₂ O₃ -MgO systemsintering aid is employed, a discussion will be made in the followingexample.

EXAMPLE 7

To Si₃ N₄ powder, were added respectively 6% of Y₂ O₃, 2% of Al₂ O₃ and1%, 3% and 5% of MgO as sintering aids to obtain three different typesof powder mixtures. They were pressure-formed under a pressure of 1000Kg/cm² and then presintered in an atmosphere of nitrogen gas attemperatures of 1550° C., 1600° C. and 1700° C. and for 100 minutes and200 minutes. Results are shown in FIG. 5.

Namely, when presintered for 200 minutes, the density of eachpresintered body becomes highest near the presintering temperature of1550° C., reaching a density above 90% despite of the application of thepressureless sintering method. When presintered for 100 minutes, thehighest density was obtained near 1600° C., where the density wasapproximately 90% or higher. In view of these results, where a Y₂ O₃-Al₂ O₃ -MgO system sintering aid is employed, it is not necessary toconduct the presintering at a high temperature of 1700° C. or higherwhich has been generally employed for sintering. It has been found thatthe presintering can be performed satisfactorily at relatively lowtemperatures. Accordingly, the presintering temperature to be used inthe present invention may preferably be in the range of 1500°-1700° C.It should be chosen from the above range in accordance with theproportion of the sintering aid. More preferably, it is desirous tolimit the presintering temperature to a range of 1500°-1600° C.

The presintering time is now described in the following example.

EXAMPLE 8

The sintering aids were incorporated in various proportions in Si₃ N₄powder. The resulting powder mixtures were formed into green compacts inmuch the same way as in Example 6. The green compacts were thereafterpresintered at normal pressure in an atmosphere of N₂ gas at 1600° C.for 25-200 minutes. Results are shown diagrammatically in FIG. 6.

As appreciated from FIG. 6, where conventional Y₂ O₃ -Al₂ O₃ systemsintering aids were employed, there is a tendency that the relativedensity becomes higher as the presintering time becomes longer. On theother hand, when Y₂ O₃ -Al₂ O₃ -MgO system sintering aids wereincorporated, an opposite trend is observed, in other words, therelative density tends to lower as the presintering time becomes longer.Accordingly, in the present invention, a particularly long sinteringtime period is unnecessary. To obtain a high density presintered bodyhaving a relative density of 92% or higher, a presintering time periodof not longer than 200 minutes is necessary. Preferably, it is desirousto conduct the presintering step in 100 minutes. On the other hand, ifthe presintering time is too short, the sintering does not proceed to asufficient degree. Thus, the presintering time is preferably at least 30minutes, more preferably, 50 minutes or longer. Therefore, where a Y₂ O₃-Al₂ O₃ -MgO system sintering aid is employed, the presintering can beconducted in a shorter period of time compared with the case in which aconventional Y₂ O₃ -Al₂ O₃ system sintering aid is used. The formersintering aid is thus expected to bring about another effect that thepresintering energy can be saved considerably.

In accordance with the above-described method, it is possible to obtaina high density presintered body having a relative density of 92% orhigher by the pressureless sintering method. It is also possible toobtain highly strong products having a bend strength of 70 Kg/mm² orhigher. However, to obtain a sintered product having a still higherdensity and strength, the application of the pressureless sinteringmethod only will encounter a limit and it is thus densified further inaccordance with the above-mentioned HIP method. Although it has alreadybeen well-known to produce high density sintered Si₃ N₄ in accordancewith the HIP method, such HIP method was generally carried out byhermetically sealing Si₃ N₄ powder or its green compact in a capsulemade of a deformable material. Glass is usually used to make suchcapsules.

However, since the density of Si₃ N₄ powder or its green compact to besealed in a capsule is extremely low, the capsule undergoes a highdegree of deformation during the HIP treatment, whereby resulting in thebreakage of the capsule and making the HIP treatment fruitless. Even ifthe capsule barely escapes breakage, a difference in shrinkage tends tooccur between a thick portion and thin portion and leads to poordimensional accuracy. Where the capsule is made of glass, the glass isfused at high temperatures and penetrates into gaps between Si₃ N₄powder particles, thereby unavoidably lowering the heat-resistantproperties of a resultant sintered product. There is also anotherdrawback that cracks may occur in the sintered product due to thedifference in thermal expansion coefficient between the sintered Si₃ N₄and glass while the sintered Si₃ N₄ is cooled after the HIP treatment.

On the other hand, in the present invention, the relative density of agreen compact is increased to a value of 92% or higher through theabove-mentioned pressureless presintering method. Thus, in the resultingpresintered body, most of pores are closed. This permits to subject thepresintered body directly to an HIP treatment without using a capsule asrequired in the conventional method. In carrying out the HIP treatment,Ar gas is generally employed. However, in the present invention, it isnecessary to use N₂ gas, which is capable of suppressing thedecomposition of Si₃ N₄, as a pressure medium gas since the presinteredbody is exposed directly in the pressure medium gas without protectingthe same in a capsule and Si₃ N₄ would undergo thermal decomposition athigh temperatures, thereby leading to deterioration of the surface layerof the sintered product and lowering of its density.

The HIP method of the present invention will be described further in thefollowing example.

EXAMPLE 9

The presintered bodies of Sample Nos. 4 through 7 in Example 6 weresubjected to an HIP treatment for 60 minutes in an atmosphere of N₂ gasof 1700°-1900° C. and 1000 atm. A measurement of the density and bendstrength at room temperature of each of the resultant sintered productsgave results shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Before HIP    HIP conditions                                                                              After HIP                                         No.  R.D. (%)  B.S.   Temp. P.   Time R.D. (%)                                                                              B.S.                            ______________________________________                                        4    86        32     1900  1000 60   90      45                              5    93        57     1700  1000 60   98      88                              6    95        70     1700  1000 60   98      85                              7    91        44     1700  1000 60   96      71                              ______________________________________                                         Note:                                                                         R.D. -- Rleative Density;                                                     B.S. -- Bend strength (Kg/mm.sup.2)                                           Temp. -- Temperature (°C.);                                            P. -- Pressure (atm);                                                         Time (min)                                                               

As clearly appreciated from the above table, both density and strengthare considerably improved by the HIP treatment. Particularly, in SampleNos, 5 and 6 whose densities were relatively high before HIP, theirdensities were raised to as high as 98% by the HIP treatment as well astheir strengths gave considerably high values of 88 Kg/mm² and 85 Kg/mm²respectively. Thus, distinct effects of the HIP treatment areunderstood. In Sample Nos. 4 and 7 whose densities were rather lowcompared with Sample Nos. 5 and 6 before HIP, the effects of the HIPtreatment are observed low. This is explained as follow. Namely, theporosity of each presintered body was high and more pores communicatedto the surface as open pores, whereby reducing the pore-closing effectof the HIP treatment. According to various research results obtained bythe present inventors, the effect of the HIP treatment can be exhibitedclearly where the relative density of a presintered body reaches 90%. Ithas been found, particularly, that high density sintered products of arelative density of 98% or higher can be obtained substantially alwaysif the relative density of a presintered body is 92% or higher.

Accordingly, to enhance the effect of the HIP treatment, it ispreferable to prepare in advance a presintered body having a relativedensity of 92% or higher in accordance with the pressureless sinteringmethod.

As described above in detail, a presintered body of a high density canbe easily obtained in accordance with the pressureless sintering methodby using a Y₂ O₃ -Al₂ O₃ -MgO system sintering aid. Since Si₃ N₄ isformed into a desired shape beforehand in a powder state, a complexshape can be easily formed. In addition, the presintering temperaturecan be relatively lowered and the presintering time can be shortened,thereby enabling to lower the sintering cost and energy considerably.

When densifying and strengthening further the presintered body producedby the above-described pressureless sintering method in accordance withthe HIP method, the HIP treatment can be easily performed without usingany capsule and the effects of the HIP treatment can be enhanced furthersince the presintered body obtained in the above method has already beendensified to a considerable degree.

EXAMPLE 10

As a raw material powder, was used commercially available Si₃ N₄ powderproduced by nitridation of Si (content of α-Si₃ N₄ : 90%; content ofβ-Si₃ N₄ : 10%; mean particle size: 1 μm), to which were added assintering aids 6% by weight of Y₂ O₃, 3% by weight of MgO and 2% byweight of Al₂ O₃. The resulting mixture was cold compacted under apressure of 1 ton/cm² and presintered under the atmospheric pressure, inN₂ gas and for 200 minutes, by changing the sintering temperatures.Presintered bodies having properties shown in Sample Nos. 1 through 7 inTable 6 were resulted. Then, these presintered bodies were subjected toan HIP treatment under the HIP conditions indicated in Sample Nos. 1through 7 in Table 7 without applying a pore-closing treatment to them.A measurement of the properties of each of the resultant sinteredproducts gave the results summarized in Sample Nos. 1 to 7 in Table 7.In Tables 6 and 7, Sample Nos. 1-4, 6 and 7 relate to the presentinvention while Sample No. 5 is a comparative example.

                                      TABLE 6                                     __________________________________________________________________________                         Properties of presintered body                                      Sintering aid                                                                           Relative                                                                           Content of                                                                          Bend strength at                              Sample                                                                            Si.sub.3 N.sub.4                                                                     and amount                                                                              density                                                                            β-Si.sub.3 N.sub.4                                                             room temperature                              No. Powder added (wt. %)                                                                           (%)  (%)   (Kg/mm.sup.2)                                 __________________________________________________________________________     1  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           94   25    56                                                method Al.sub.2 O.sub.3 (2)                                                2  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           96   58    47                                                method Al.sub.2 O.sub.3 (2)                                                3  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           96   58    47                                                method Al.sub.2 O.sub.3 (2)                                                4  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           95   76    51                                                method Al.sub.2 O.sub.3 (2)                                                5* Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           94   85    61                                                method Al.sub.2 O.sub.3 (2)                                                6  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           96   48    62                                                method Al.sub.2 O.sub.3 (2)                                                7  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Mgo (3),                                                           97   57    50                                                method Al.sub.2 O.sub.3 (2)                                                8* Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), MgO (5),                                                           98   81    70                                                method Al.sub.2 O.sub.3 (2)                                                9  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), MgO (1),                                                           96   61    45                                                method Al.sub.2 O.sub.3 (2)                                               10  Si-nitridation                                                                       Y.sub.2 O.sub.3 (8), Al.sub.2 O.sub.3 (2)                                               93   56    69                                                method                                                                    11* Si-nitridation                                                                       Y.sub.2 O.sub.3 (8), Al.sub.2 O.sub.3 (2)                                               95   90    75                                                method                                                                    12* Si-nitridation                                                                       MgO (5),Al.sub.2 O.sub.3 (3),                                                           97   100   58                                                method Fe.sub.2 O.sub.3 (1)                                               13  Gas phase                                                                            Y.sub.2 O.sub.3 (6), MgO (3),                                                           93   35    58                                                method Al.sub.2 O.sub.3 (2)                                               14* Gas Phase                                                                            Y.sub.2 O.sub.3 (6), MgO (3),                                                           94   83    73                                                method Al.sub.2 O.sub.3 (2)                                               15  Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Al.sub.2 O.sub.3 (2)                                               83   66    35                                                method                                                                    16* Si-nitridation                                                                       Y.sub.2 O.sub.3 (6), Al.sub.2 O.sub.3 (2)                                               81   95    46                                                method                                                                    __________________________________________________________________________     *Comparative example                                                     

                                      TABLE 7                                     __________________________________________________________________________    Pore-    HIP treatment conditions                                                                        Properties of sintered body                        Sample                                                                            closing                                                                            Temp.                                                                             Pressure                                                                           Time                                                                              Pressure                                                                           Relative                                                                            Content of                                                                           Bend strength at                      No. treatment                                                                          (°C.)                                                                      (atm)                                                                              (min)                                                                             gas  density (%)                                                                         β-Si.sub.3 N.sub.4                                                              room temp. (Kg/mm.sup.2)              __________________________________________________________________________     1  Skipped                                                                            1700                                                                              1000 60  N.sub.2                                                                            98    95     81                                     2  Skipped                                                                            1700                                                                               500 60  N.sub.2                                                                            99    75     82                                     3  Skipped                                                                            1700                                                                              1000 60  Ar   95    99     63                                     4  Skipped                                                                            1700                                                                              1700 60  N.sub.2                                                                            100   79     87                                     5* Skipped                                                                            1700                                                                              1000 60  N.sub.2                                                                            99    100    48                                     6  Skipped                                                                            1850                                                                              1000 60  N.sub.2                                                                            100   100    95                                     7  Skipped                                                                            1850                                                                              1700 60  N.sub.2                                                                            100   100    73                                     8* Skipped                                                                            1700                                                                              1000 60  N.sub.2                                                                            99    82     73                                     9  Skipped                                                                            1700                                                                              1000 60  Ar   95    100    88                                    10  Skipped                                                                            1700                                                                              1000 60  N.sub.2                                                                            98    100    98                                    11* Skipped                                                                            1700                                                                              1000 60  N.sub.2                                                                            99    100    60                                    12* Skipped                                                                            1850                                                                              1000 60  N.sub.2                                                                            99    100    58                                    13  Skipped                                                                            1700                                                                              1000 30  N.sub.2                                                                            99    98     108                                   14* Skipped                                                                            1700                                                                              1000 30  N.sub.2                                                                            96    98     62                                    15  Applied                                                                            1800                                                                               700 60  N.sub.2                                                                            95    100    85                                    16* Applied                                                                            1800                                                                               700 60  N.sub.2                                                                            93    100    49                                    __________________________________________________________________________     *Comparative example                                                     

As readily appreciated from the results given in the above tables,except for Sample No. 5 which is a comparative example, the content ofβ-Si₃ N₄ in the presintered body of each of Sample Nos. 1-4, 6 and 7which were prepared in accordance with the present invention ranges25-76%. In each of Sample Nos. 1-4, 6 and 7, the relative density wasimproved by the HIP treatment and the bend strength was considerablyimproved.

On the other hand, in Sample No. 5 in which the content of β-Si₃ N₄ inthe presintered body exceeds 80%, the bend strength was rather loweredby the HIP treatment although its relative density was increased by thesame HIP treatment.

In order to investigate influence of the type of pressure medium gas inthe HIP treatment, Sample No. 1 in which N₂ gas was used as a pressuremedium gas was compared with Sample No. 3 in which Ar gas was employedas a pressure medium gas. The relative density was not improved by theHIP treatment where Ar gas was used, although the bend strength wasapparently improved. When N₂ gas was employed, both relative density andbend strength were improved by the HIP treatment. Thus, it is easilyappreciated that N₂ gas is a preferred pressure medium gas.

EXAMPLE 11

Similar to Example 10, Si₃ N₄ powder was employed. By varying the typeand amount of sintering aids as indicated in Sample Nos. 8-12 in Table6, Sample Nos. 8, 9 and 12 were compacted and presintered in much thesame way as in Example 10. Sample Nos. 10 and 11 were presinteredthrough hot pressing respectively at 1600° C. and 1700° C., at 250Kg/cm² and for 30 minutes, resulting in the production of presinteredbodies having properties as shown in Sample Nos. 8-12 in Table 6. Thesepresintered bodies were, similar to Example 10, subjected to an HIPtreatment under the HIP conditions given in Sample Nos. 8-12 in Table 6.Thereafter, the properties of the resultant sintered products weremeasured, providing results as shown in Sample Nos. 8-12 in Table 7. InTables 6 and 7, Sample Nos. 8, 11 and 12 represent comparative examples.

It is understood, from the results summarized in the above tables, that,although Sample Nos. 9 and 10 used sintering aids different from thoseemployed in Example 10, the HIP treatment could improve their relativedensities and bend strengths similar to Example 10.

On the other hand, in Sample Nos. 8, 11 and 12 in which the contents ofβ-Si₃ N₄ in the presintered bodies were high, namely, 81%, 90% and 100%respectively, their relative densities were increased by the HIPtreatment whereas improvement in bend strength was hardly observed.Thus, it is appreciated that, even if the content of β-Si₃ N₄ in apresintered body is raised to 100%, the bend strength of a sinteredproduct to be obtained through an HIP treatment would not be improved.

EXAMPLE 12

As a raw material Si₃ N₄ powder, was employed Si₃ N₄ powder prepared inaccordance with the gas phase method (amorphous portions: 40%; α-Si₃ N₄: 57%; β-Si₃ N₄ : 3%; mean particle size: 1 μm), to which the samesintering aids as in Example 10 were added and mixed. The resultantpowder mixture was compacted and presintered similar to Example 10,resulting in the production of presintered bodies having propertiesgiven in Sample Nos. 13 and 14 in Table 6. Then, these presinteredbodies were subjected to an HIP treatment under the HIP conditions shownin Sample Nos. 13 and 14 in Table 7, similar to Example 10. Ameasurement of the properties of each of the resulting sintered productsgave results shown in Sample Nos. 13 and 14 in Table 7. In Tables 6 and7, Sample No. 14 represents a comparative example. From the results inthese tables, it is easily appreciated that Sample No. 13, in which thecontent of β-Si₃ N₄ in the presintered body was 35% was considerablyimproved in bend strength through its HIP treatment compared with SampleNo. 14, in which the content of β-Si₃ N₄ accounted for 83%, and themethod of this invention thus has a great effect in improving the bendstrength of a sintered product even if Si₃ N₄ powder made by the gasphase method is employed.

It is also appreciated from the above tables that the bend strength canbe improved by the HIP treatment more effectively when Si₃ N₄ powderobtained by the gas phase method is employed as a starting material thanwhen Si₃ N₄ powder produced by the nitridation method is used as astarting material.

EXAMPLE 13

The same Si₃ N₄ as that used in Example 10 was used. It was then addedwith the sintering aids shown in Sample Nos. 15 and 16 in Table 6 andmixed therewith. Similar to Example 10, the thus-obtained powdermixtures were compacted and presintered into presintered bodies, whoseproperties are given in Sample Nos. 15 and 16 in Table 6. Thesepresintered bodies were immersed in an organic solvent slurry containinga powder mixture which consists of 80% by weight of Si₃ N₄, 10% byweight of SiO₂ and 10% by weight of Al₂ O₃, thereby applying the slurryon the surfaces of the bodies, drying and firing the same. Now, theirpore-closing treatments were completed. Then, similar to Example 10,they were subjected to an HIP treatment under the treatment conditionsgiven in Sample Nos. 15 and 16 in Table 7. A measurement of theproperties of each of the resultant sintered product gave results asshown in Sample Nos. 15 and 16 in Table 7. In Tables 6 and 7, Sample No.16 is a comparative example.

From the above tables, it is understood that a greater improvement bythe HIP treatment to the bend strength was achieved in Sample No. 15containing β-Si₃ N₄ in a proportion of 66% compared with Sample No. 16which contained β-Si₃ N₄ as much as 95% and the method of this inventioncan exhibit a great effect in improving the bend strength of a sinteredproduct even if the HIP treatment is conducted on a presintered bodywhich has been subjected in advance to a pore-closing treatment.

Putting all the results obtained in the above Examples 10 to 13together, it is clear that, according to this invention, both relativedensity and bend strength can be improved to a considerable extent andsintered Si₃ N₄ products of a high density and strength can be prepared.

EXAMPLE 14

In commercially available Si₃ N₄ powder produced by nitrogenizing Sipowder, were incorporated in varied proportions Y₂ O₃ powder, Al₂ O₃powder and MgO powder as sintering aids. The thus-prepared powdermixtures were compacted under a pressure of 500 Kg/cm² and, then,resulting green compacts were respectively presintered in an atmosphereof N₂ gas under the presintering conditions given in Table 8, followedby charging the thus-obtained presintered bodies into an HIP furnace attemperatures shown in Table 8. They were then subjected to an HIPtreatment under the HIP conditions, also given in Table 8. Resultantsintered products were withdrawn out of the HIP furnace at temperaturesindicated in Table 8, followed by the application of a heat treatmentgiven in the same table, resulting in the preparation of sintered Si₃ N₄products.

A measurement of density and bend strength on each of these Si₃ N₄products gave results shown in Table 8.

                                      TABLE 8                                     __________________________________________________________________________                   Presintering                                                                          Charging                                                                           HIP treatment                                                                             Discharging                               Sintering aid                                                                            conditions                                                                            Temp.                                                                              Conditions  temp. from                            Sample                                                                            Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --MgO                                                 Temp                                                                              Time                                                                              into HIP                                                                           P   Temp.                                                                             Time                                                                              HIP furnace                           No. (%)        (°C.)                                                                      (min)                                                                             (°C.)                                                                       (atm)                                                                             (°C.)                                                                      (min)                                                                             (°C.)                          __________________________________________________________________________    1   6, 2, 3,   1600                                                                              100 1000 1000                                                                              1700                                                                              30  1100                                  2   6, 2, 3,   1600                                                                              100 1000 1000                                                                              1700                                                                              30  1100                                  3   6, 2, 3,   1600                                                                              100  300 1000                                                                              1700                                                                              30  1100                                  4   6, 2, 3,   1600                                                                              100  700 1000                                                                              1700                                                                              30   800                                  5   6, 2, 3,   1600                                                                              100  700 1000                                                                              1700                                                                              30   500                                  6   6, 2, 3,   1600                                                                              100 1000  700                                                                              1700                                                                              30  1100                                  7   6, 2, 3,   1600                                                                              100 1000 1000                                                                              1700                                                                              30  1100                                  8   6, 2, 3,   1600                                                                              100 --   --  --  --  --                                    9   6, 2, 5,   1550                                                                              200 1000 1000                                                                              1800                                                                              10  1100                                  10  6, 2, 5,   1550                                                                              200 --   --  --  --  --                                    11  6, 2, 1,   1550                                                                              200 1000 1000                                                                              1800                                                                              10  1100                                  12  6, 2, 1,   1550                                                                              200 --   --  --  --  1100                                  __________________________________________________________________________        Charging temp.                                                                         Heat treatment                                                                          Properties of sintered body                                into heat tre-                                                                         conditions                                                                              Relative                                                                            Bend                                             Sample                                                                            atment furnace                                                                         Temp Time density                                                                             strength                                         No. (°C.)                                                                           (°C.)                                                                       (min)                                                                              (%)   (Kg/mm.sup.2)                                                                        Note                                      __________________________________________________________________________    1   1000     --        99    43     Comparative example                       2   1000     1000 5    99    78                                               3   1000     1000 5    99    46     Comparative example                       4    700      700 5    99    83                                               5    400      400 5    99    37     Comparative example                       6   1000     1000 5    99    84                                               7   1000     1400 300  99    109                                              8   --       --        92    50     Comparative example                       9   1000      800 5    99    89                                               10  --       --        95    64     Comparative example                       11  1000      800 5    98    75                                               12  1000     --        85    41     Comparative example                       __________________________________________________________________________

As apparent from Table 8, in Comparative Example No. 1 wherein no heattreatment was effected subsequent to the HIP treatment and ComparativeExample No. 5 wherein the heat treatment temperature was low, namely,400° C., the cooling velocity of each of the sintered bodies after theHIP treatment was fast. Thus, their bend strengths were low althoughthey were highly densified. On the other hand, Sample Nos. 2, 4 and 6were each a sintered product having a high density as well as high bendstrength. It has thus recognized that the heat treatment is veryeffective. This effect appears to have been derived from the fact thatfine cracks occurred in Comparative Example Nos. 1 and 5 due to thethermal shocks caused by the sudden cooling after the HIP treatmentwhereas, in Sample Nos. 2, 4 and 6, the sintered bodies were not rapidlycooled down owing to the incorporation of the heat treatment andoccurrence of cracks was thus avoided. In addition, Sample No. 7, whichwas subjected to a longer heat treatment, i.e., a heat treatment for 5hours had an outstandingly high bend strength compared with the rest ofthe samples because the glassy phases at boundaries of Si₃ N₄ had beencrystallized in the course of the heat treatment.

On the other hand, Comparative Example No. 3 which was charged at a lowtemperature, i.e., 300° C. into the HIP furnace had a low bend strengthand was much inferior in bend strength to other samples. In ComparativeExample No. 3, the bend strength of the final sintered product isconsidered to have been weakened, probably, as a result that structuraldefects such as cracks occurred by thermal shocks due to the rapid uponthe HIP treatment.

Similar tendency was also observed on Sample Nos. 9, 10, 11 and 12 inwhich the amounts of the sintering aids were changed. Sample Nos. 9 and10 were improved in both density and bend strength compared withpresintered bodies, to which no HIP treatment was applied and, asenvisaged, had good density and bend strength.

By the way, it is clear that in each of the above-described methodsretention of temperature in each of the steps above a requiredtemperature conforms with the view point of energy saving andcontributes considerably to the saving of energy.

In view of the above results, it can be concluded that the method ofthis invention is extremely effective from industrial standpoint as aproduction method of high density sintered silicon nitride(Si₃ N₄). Itsindustrial application is expected in the near future, particularly, asa method for producing high density sintered Si₃ N₄ of complexconfigurations since the HIP treatment is effected without usingcapsules.

Although the invention has been described with reference to preferredembodiments, other embodiments may be resorted to without departing fromthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for producing high density sinteredsilicon nitride (Si₃ N₄) having a relative density of at least 98%, saidmethod comprising:forming silicon nitride powder into a desired shape toobtain a silicon nitride green compact; presintering said green compactunder a nitrogen gas atmosphere ranging in pressure from 1 to 30atmospheres in a presintering furnace at an elevated temperature into apresintered body having a relative density of at least 92%; dischargingsaid presintered body at a temperature of at least 500° C. from saidpresintering furnace; immediately charging said presintered body at atemperature of 500° C. or higher into a hot isostatic pressing furnacewhich has been preheated to 500° C. or higher in advance and subjectingsaid presintered body to hot isostatic pressing without the body beingconfined by a capsule in an inert gas atmosphere of a temperature in therange of 1500°-2100° C. and of a nitrogen gas partial pressure of atleast 500 atm until the former relative density is reached; dischargingthe sintered body at a temperature of at least 500° C. from the hotisostatic pressing furnace; and then charging said sintered body at atemperature of 500° C. or higher into a heat treatment furnace, whichhas been maintained at 500° C. or higher, to conduct a heat treatment ina non-oxidizing gas atmosphere.
 2. The method as claimed in claim 1,wherein said silicon nitride powder contains at least 80% by weight ofα-Si₃ N₄.
 3. The method as claimed in claim 1, wherein said siliconnitride powder further contains a sintering aid.
 4. The method asclaimed in claim 3, wherein said sintering aid is one or more compoundsselected from the group consisting of Y₂ O₃, Al₂ O₃, MgO, ZrO₂, TiO₂,BeO, La₂ O₃, CeO₂, TiN, and AlN and is contained in a total amount ofnot more than 30% by weight.
 5. The method as claimed in claim 3 or 4,wherein said sintering aid is a Y₂ O₃ -Al₂ O₃ -MgO system powdermixture.
 6. The method as claimed in claim 5, wherein said siliconnitride powder contains 3-13% of Y₂ O₃, 0.5-4% of Al₂ O₃ and 0.5-6% ofMgO, all by weight.
 7. The method as claimed in claim 1, wherein saidpresintered body contains 20-80% by weight of β-Si₃ N₄ and the contentof β-Si₃ N₄ in the sintered silicon nitride (Si₃ N₄) is increased to 80%or higher through the hot isostatic pressing.
 8. The method as claimedin claim 7, wherein the content of β-Si₃ N₄ in the sintered siliconnitride (Si₃ N₄) is increased to 90% or higher through the hot isostaticpressing.
 9. The method as claimed in claim 1, wherein the presinteringtemperature is 1400°-1800° C.
 10. The method as claimed in claim 1,wherein said presintered body is buried in powder consisting principallyof at least one nitride ceramics selected from the group consisting ofsilicon nitride, aluminum nitride and boron nitride and is thensubjected to the hot isostatic pressing.
 11. The method as claimed inclaim 1, wherein the hot isostatic pressing is carried out at atemperature of 1700°-2000° C.
 12. The method as claimed in claim 1,wherein the hot isostatic pressing is carried out for a period of fromminute to 3 hours.
 13. The method as claimed in claim 1, wherein thenitrogen partial pressure during the hot isostatic pressing ismaintained at 700 atm or higher.
 14. The method as claimed in claim 1,wherein the nitrogen partial pressure during the hot isostatic pressingis maintained at 2500 atm or lower.
 15. The method as claimed in claim1, wherein the hot isostatic pressing is carried out at a temperaturehigher than the presintering temperature.
 16. The method as claimed inclaim 1, wherein grain boundaries of the sintered silicon nitride(Si₃N₄) are crystallized in the final heat treatment step.
 17. The method asclaimed in claim 1, wherein grain boundaries of the sintered siliconnitride (Si₃ N₄) are crystallized in the hot isostatic pressing step.18. The method as claimed in claim 1, wherein said silicon nitridepowder has been obtained in accordance with the gas phase reactionmethod or thermal decomposition method.
 19. The method as claimed inclaim 1, wherein the hot isostatic pressing is carried out at atemperature lower by at least 100° C. than the decomposition temperatureof Si₃ N₄ at the pressure employed in the hot isostatic pressing step.20. The process as claim 1, wherein the silicon nitride powder fromwhich the presintered body is made is a synthetic silicon nitride powderprepared by reacting SiCl₄ with NH₃.
 21. A method for producing highdensity sintered silicon nitride (S₃ N₄) having a relative density of atleast 98%, said method comprising:forming silicon nitride powder into adesired shape to obtain a silicon nitride green compact; presinteringsaid green compact under a nitrogen gas atmosphere ranging in pressurefrom 1 to 30 atmospheres in a presintering furnace at an elevatedtemperature into a presintered body having a relative density of atleast 92%; cooling said presintered body to room temperature; chargingsaid presintered body into a hot isostatic pressing furnace which hasbeen preheated to 500° C. or higher in advance and subjecting thepresintered body to hot isostatic pressure without being confined by acapsule in an inert gas atmosphere of a temperature ranging from1500°-2100° C. and of a nitrogen gas partial pressure of at least 500atm until the former relative density is reached; discharging thesintered body at a temperature of at least 500° C. from the hotisostatic pressing furnace; and then charging the sintered body at atemperature of 500° C. or higher into a heat treatment furnace, whichhas been maintained at 500° C. or higher, to conduct a heat treatment ina non-oxidizing gas atmosphere.